In a video special effects system, video mixer circuits are used to combine two video signals into a single signal in response to a control signal. In the simplest type of mixer, the control signal is either on or off, sometimes called a binary key signal, and this on/off signal determines which of the two input signals passes to the output at any given instant.
Another type of mixer is the linear mixer or linear keyer. In this case, the control signal may assume any value between full on and full off, and the resulting video output signal will be a mixture of the two input signals based on the instantaneous voltage of the control signal. For example, if the mixer selects input A when the control signal is low and input B when the control signal is high, slowly raising the control voltage from zero to full on will result in a cross fade or dissolve from input A to input B.
These control signals are usually analog signals; however in a digital system, the control signal is often generated by digital circuitry as a binary numerical value. In this case the numerical data is passed through a digital to analog converter to produce the linear control voltage used by the linear keyer.
In a digital controlled attenuator, sometimes used in audio hybrid digital/analog systems, a digital control word determines the amount of attenuation of a single signal attached to the input. However, in a hybrid digital/analog video system, it is desired to mix two input signals in proportion and to control the proportion using a digital control word.
It is therefore an object of this invention to produce a linear keyer which responds directly to numerical control data in binary form and uses this data to proportionally combine two video signals.
Other objects and features of the invention and the manner in which the invention achieves its purpose will be appreciated from the following description and the accompanying drawings which exemplify the invention, it being understood that changes may be made in the specific method and apparatus disclosed herein without departing from the essentials of the invention set forth in the appended claims.